The present invention relates generally to spacecraft propulsion and more particularly to ion propulsion engines for use on a spinning spacecraft.
The concept of using an ion source to provide propulsion for a spacecraft has been known for a long time. Two early patents in this area are U.S. Pat. No. 3,052,088 to Davis et al and U.S. Pat. No. 3,156,090 to Kaufman, each of which describes an electrostatic ion propulsion device. However, almost all propulsion systems currently used for planetary or Earth orbital applications are chemical thrusters. There is an increasing need for thrusters providing the advantages of electrostatic ion thrusters because of an increasing demand for extended lifetime of orbital satellites. While satellite components can be made to function for extended operational periods in space, a limiting factor for satellite lifetime has been the depletion of stationkeeping propellant necessitating satellite retirement. The primary use of stationkeeping propellant is for north-south stationkeeping which must compensate for gravitational forces of the sun and moon which tend to increase the inclination of a geosynchronous orbit. For a given satellite, replacing the chemical liquid bipropellant with ion propulsion system hardware and propellant can as much a double the operational lifetime of the satellite. This is possible because of the very high specific impulse provided by an ion thruster as compared to chemical thrusters and because the power source for the ion thruster is the sun.
An ion thruster produces thrust by expelling propellant ions at high exhaust velocity. Typically the exhaust velocity and thus the specific impulse is selected on the basis of mission requirements for velocity increment, power available, and propellant requirement. There is usually an optimum value for the specific impulse. For a given exhaust velocity, thrust level is usually adjusted by varying the ion beam current, i.e. the number of ions ejected per unit time. For attitude control of a spinning satellite, large variations in ion beam current would be required to be accomplished in a relatively short time and this is not tractable for currently available ion thruster systems.
The idea of powering an ion propulsion subsystem for north-south stationkeeping (NSSK) from the batteries which are used on geosynchronous communication satellites for operation during eclipse periods was first suggested by B. A. Free in "Electric Propulsion for Communications Satellites," COMSAT TM CL-4-80, COMSAT Laboratories, January 1980. It has since been shown that there can be substantial economic benefits for communication satellites by reducing the amount of chemical propellant which must be carried through the installation of a high specific impulse ion propulsion subsystem to perform NSSK. R. L. Poeschel, "Ion Propulsion for Communications Satellites," JSA SS/AIAA/ DGLR Paper No. IEPC 84-43, Tokyo, Japan, May 1984.
In previously developed ion propulsion subsystems, mercury has been used for propellant because its high atomic mass results in a very high specific impulse (Isp), on the order of 3,000 seconds. However, the inert gas xenon provides performance very near that of mercury while resulting in several major simplifications in propellant handling.
It is desirable with a spin-stabilized satellite to have the thruster or thrusters for NSSK located off-axis to allow both velocity correction and spin-axis attitude correction by spin-synchronous modulation of the thrust. The use of off-axis chemical thrusters with on-off pulsing for attitude and velocity control is described in U.S. Pat. No. 3,758,051, to Williams. Thrust modulation or off-pulsing is also important for dual-spin satellites having a center of mass which is off the spin axis because it allows the spin-period averaged thrust to be directed through the spacecraft center of mass.
Off-pulse thrust modulation of ion propulsion thrusters presents problems not present with chemical thrusters. There is a significant start-up time, on the order of several minutes, required for these thrusters. Additionally, electromechanical modulation of the propellant flow rate into the plasma region is undesirable because of difficulty with precise control and repeated operation of mechanical valves reduces reliability.